This description relates to high voltage direct current (HVDC) transmission systems, and, more particularly, to HVDC converter systems and a method of operation thereof.
High voltage direct current (HVDC) electrical power transmission, in contrast with the prevalent alternating current (AC) systems, exhibits benefits of cost and loss reduction in the long-distance transmission of electrical power. Throughout the world, electrical power is traditionally distributed with high voltage AC. High voltage is used for transmission because the loss of energy during transmission is proportional to the amount of electrical current squared (I2R losses). Thus, raising the voltage instead of raising the current allows more power to be transmitted without significantly increasing the transmission losses. AC was selected for power transmission in the early days of electrification instead of direct current (DC) because it was easier to transform between different voltage levels with alternating current than with direct current.
Although alternating current is used for most electrical energy transmission, alternating current has its own set of problems. Alternating current generally requires more conductors to carry a similar amount of electrical power than direct current. Alternating current can suffer a ‘skin effect,’ where much of the power transmission is carried by the outer surface of the conductor instead of being uniformly carried by the conductor, thus resulting in increased transmission losses as compared to direct current transmission. Furthermore, it can be very difficult to transmit electrical power with alternating current with undersea or underground cables due to the associated increased cable capacitance. Thus, for many long-distance electrical energy transmission tasks, high-voltage direct current is used instead of alternating current. Direct current transmission is also able to connect asynchronous power grids through a DC hub, which permits a controlled flow of energy while also functionally isolating the independent AC frequencies of each side and thereby reducing fault propagation through the AC-DC-DC interconnection. For example, the Eastern Interconnection and Western Interconnection are the two major alternating-current (AC) electrical grids in North America. The Texas interconnection is one of several minor interconnections with respect to the Eastern and Western Interconnection. The Eastern, the Western and the Texas Interconnections may be connected via HVDC interconnection links. Geographically overlapping but electrically isolated asynchronous grids can be connected for the same purpose as above using “Back-to-Back” converters configuration requiring no additional transmission lines. Back-to-Back HVDC system can be seen as a specific case of HVDC transmission system.
HVDC transmission lines can also carry electrical energy over long distances with transmission losses significantly less than alternating current transmission losses. For example, high-voltage direct current transmission line losses are typically 30 to 40% lower than alternating current transmission line losses at the same voltage levels. Alternating current transmission lines are limited by their peak voltage levels but do not transmit much power at those peak levels, whereas direct current can transmit full power at the peak voltage level. Furthermore, as described above, because direct current does not involve multiple phases nor suffers from the skin effect, direct current transmission lines can have fewer conductor lines and smaller conductor lines. As a result, a dimension and costs of the transmission towers is reduced along with issues associated with the right-of-way. Additionally, reactive power issues that affect alternating current transmission do not affect direct current transmission.
However, HVDC transmission is generally avoided unless the power is being transmitted by an undersea cable or over a very long distance. High-voltage direct current is generally avoided because the conversion equipment is very complex and expensive. Thus, even though direct current provides significant efficiency advantages for electrical energy transmission, direct current is not as often used for electricity transmission as alternating current. At least some known HVDC transmission systems include conventional conversion equipment that typically includes a multi-phase AC-to-DC converter, a long distance DC power conductor, such as, but not limited to an electrical cable for transmission of the electrical power, and a multi-phase DC-to-AC inverter on the load end of the system. Switching valves in the multi-phase AC-to-DC converter and multi-phase DC-to-AC inverter are typically silicon-based and subject to relatively low voltage and current ratings. To increase the ratings of the system to a level useful for power transmission systems, many such valves are coupled in electrical series and/or electrical parallel. Although such connections increase the ratings of the multi-phase AC-to-DC converter and the multi-phase DC-to-AC inverter, such connections also increase the complexity of commutation of the valves, the complexity of the current and voltage sharing among the valves, and the space requirements of the components that make up the conversion system.
The state-of-the-art of HVDC converter technology has evolved from the line commutated converter (LCC) HVDC using thyristors, to voltage source converter (VSC) HVDC, in which output voltage polarity does not change, and in which self-commutating power semiconductors such as IGBTs or IGCTs are used. VSC HVDC has evolved, most recently to the modular multilevel converter (MMC) HVDC technology and their hybrid combinations. The LCC HVDC is a current source system, meaning that in which the current direction does not change and in which the thyristor is used. Because the thyristor, unlike IGBTs or IGCTs, cannot be forced to turn off, this type of system relies on the grid for commutation. Therefore, unlike VSC HVDC it consumes a large amount of reactive power compensation as well as requires a strong grid environment. VSC HVDC on other hand is fully capable of operating in weak AC systems, have “Black” start capability without requiring support of external devices such as bulky and expensive synchronous condensers that would be needed for LCC HVDC in such applications. Moreover VSC converters due to their ability to independently control reactive power generate insignificant level of harmonics and hence no filters are required. As a result land area required for VSC HVDC station can be as much as 50-60% that of for LCC. However LCC systems are cost effective and continue to be used particularly for overhead transmission if the connecting grids are strong and/or where magnitude power to be transmitted well exceeds power handling capabilities of VSC systems, due principally to IGBTs or IGCTs current ratings which is much lower than for thyristors.
Advantages of VSC HVDC is enhanced by replacing power semiconductors with the gas tubes, which can be designed to manage much higher magnitudes of current, enabling therefore in much higher power ratings for VSC HVDC stations. Combination of the gas tube's significantly higher voltage ratings, in hundreds of kilovolts (kV) as compared to a few kV for the power semiconductors, and extremely fast turn on and turn off characteristics, result in considerable reduction in the number of switching elements as well as the complexity of topology to simpler two-level converter. As a consequence the system's complexity, size, weight, and costs are greatly reduced.